1. Field of the Invention
The present invention relates to a cleaning solution for semiconductor substrate and a cleaning method using said cleaning solution. More particularly, the present invention relates to a cleaning solution used for removing the platinum group metal (e.g. Pt or Ir) contaminants adhering on the insulating film formed on a semiconductor substrate, as well as to a cleaning method using said cleaning solution.
2. Description of the Related Art
In recent years, memory cells such as DRAM and the like have become increasingly finer. Meanwhile, memory cells using a conventional dielectric film such as nitride film or oxide film are unable to have a sufficient capacity because such a dielectric film has a dielectric constant of only about 2 to 3. To solve this problem, a ferroelectric film of perovskite type, made of barium titanate, strontium titanate, barium strontium titanate or the like has come to be used. When such a ferroelectric film is in direct contact with a substrate, the substrate is oxidized, making large the capacity of memory cell. Therefore, in using such a ferroelectric film, it has become common to use a lower-level capacitive electrode made of a platinum group metal (e.g. Pt or Ir) not reactive with the ferroelectric. An example of use of a lower-level capacitive electrode made of a platinum group metal is shown in FIG. 3 illustrating a key portion sectional view of a semiconductor memory device wherein data storage is made by charge accumulation using a combination of a MOS transistor and a capacitive part.
In FIG. 3, a capacitive part 12 is constituted by a ferroelectric film 10, an upper-level platinum thin film 11 and a lower-level platinum thin film 9. The lower-level platinum thin film 9 provided beneath the ferroelectric film 10 is connected to one of diffusion layers 2 of a MOS transistor via a vertical interconnect 6, and the upper-level platinum thin film 11 provided on the ferroelectric film 10 is connected to a metal wiring 13. The MOS transistor (including diffusion layers 2 and a gate electrode 4 formed on a channel region between the diffusion layers 2, via a gate insulating film 3) provided on a silicon substrate 1 is covered with a first inter-layer insulating film 5; and the capacitive part 12 formed on the first inter-layer insulating film 5 is covered with a second inter-layer insulating film 7. In the opening formed in the second inter-layer insulating film 7, the upper-level platinum thin film 11 is connected to the metal wiring 13; and the metal wiring 13 and the second inter-layer insulating film 7 are covered with a third inter-layer insulating film 8.
In producing a semiconductor memory device as shown in FIG. 3, when the second inter-layer insulating film 7 is formed, for example, so as to cover the capacitive part 12, platinum atoms or particles vaporize from the upper platinum thin film 11 of the capacitive part 12 and remain in the atmosphere gas inside the apparatus used for formation of second inter-layer insulating film or on the inner wall of the apparatus; these platinum atoms or particles adhere on the formed second inter-layer insulating film 7 or on the back side of the silicon substrate 1 and, when left as they are, give very harmful effects on the properties of the semiconductor memory device produced. When the above-mentioned apparatus once used for formation of second inter-layer insulating film is again used for formation of insulating film on silicon substrate for other semiconductor memory device, similar problems arise because the platinum atoms or particles remaining in the apparatus come to adhere on the insulating film or on the back side of the silicon substrate. It is known that remaining of platinum atoms or particles of about 1xc3x971010 atoms/cm2 gives an adverse effect on the life and electrical properties of the semiconductor memory device produced.
Such contaminants include (1) metal ions adsorbed by or bonded with the surface of a semiconductor substrate or the surface of the insulating film (e.g. silicon oxide film) formed on a semiconductor substrate and (2) metal particles adhering on said surface.
In production of a semiconductor memory device, there is a case that one common cleaning bath is used in an ordinary cleaning operation after the step for formation of platinum group metal film and in an ordinary cleaning operation after other step. In such a case, when the cleaning bath is used for cleaning of a substrate contaminated with a platinum group metal and successively used for cleaning of other substrate, secondary contamination may take place in the other substrate. Therefore, it is necessary that the platinum group metal contaminants are beforehand removed from the former substrate. Such handling of a contaminated substrate and other substrate in one common apparatus is conducted in various steps of semiconductor memory device production, such as oxide film formation step in addition to cleaning step.
As conventional metal-removing solutions, there are known, for example, hydrochloric acid-hydrogen peroxide-water (HClxe2x80x94H2O2xe2x80x94H2O, i.e. HPM), sulfuric acid-hydrogen peroxide-water (H2SO4xe2x80x94H2O2xe2x80x94H2O, i.e. SPM), nitric acid and hydrochloric acid (aqua regia), and ammonia water-hydrogen peroxide-water (NH4OHxe2x80x94H2O2xe2x80x94H2O, i.e. APM). These conventional metal-removing solutions, however, are generally for removal of heavy metals and unable to sufficiently remove the contaminants of Pt, Ir or the like having a very low ionization tendency, and can hardly reduce them to a level lower than the above-mentioned level, i.e. 1xc3x971010 atoms/cm2. Even if the contaminants could be removed from the substrate surface, the removed contaminants suspend in the cleaning solution used and readhere on the substrate when the substrate is pulled up from the cleaning solution bath.
Techniques of removing metal contaminants present on silicon wafer or in natural oxide film by using a mixed cleaning solution of hydrochloric acid, hydrofluoric acid and aqueous hydrogen peroxide are disclosed in, for example, JP-A-3-228327 and JP-A-8-31781. All of these techniques, however, are for removal of contaminants present on a silicon wafer on which no device element is formed yet. In JP-A-3-228327 is shown an example of using a cleaning solution of HF:HCl:H2O2:H2O=1:10:20:100 and conducting cleaning at room temperature; in JP-A-8-31781is shown an example of using a cleaning solution obtained by diluting a 1:1 mixture of 17 wt. % HCl and 25 wt. % HF, with a 100-fold amount of water and then adding thereto H2O2. Both of these techniques are effective for removal of ordinary contaminants, but none of these techniques suggest any effect on the removal of platinum group metal (e.g. Pt or Ir) contaminants present on silicon-based insulating film.
In JP-A-7-45580 is disclosed a technique of removing the metal (e.g. Cu) contaminants adhering on a wafer surface by a sequential washing process, i.e., first removing a natural oxide film formed on a silicon wafer, with dilute hydrofluoric acid, then treating the wafer with a mixture of hydrofluoric acid, hydrochloric acid, hydrogen peroxide and water, and further washing the wafer with a mixture of hydrochloric acid, hydrogen peroxide and water.
In this technique, in the first treatment with dilute hydrofluoric acid, etching of natural oxide film takes place and, simultaneously therewith, the platinum group metal (e.g. Pt or Ir) contaminants present on oxide film, etc. are taken into the hydrofluoric acid solution and suspended therein (these contaminants are stable to hydrofluoric acid); therefore, these contaminants readhere on the wafer when the wafer is pulled up from a cleaning bath, and no sufficient effect for contaminant removal is obtained.
Also in JP-A-6-333898 is disclosed a technique of cleaning the surface of a semiconductor substrate by using a cleaning solution containing (1) a strong acid and an oxidizing agent both for removal of the organic substances and inorganic substances present on semiconductor substrate, (2) a fluorine-containing compound consisting of fluorosulfuric acid or sulfuryl difluoride, capable of releasing fluorine for very slight etching of semiconductor substrate and consequent removal of residue and particles present on semiconductor substrate, and (3) water. In the Example of the document is shown a case of removing a residue adhering on a polysilicon film after dry etching. However, no mention is made on the removal of platinum group metal contaminants.
The object of the present invention lies in providing (1) a cleaning solution which can reliably remove the platinum group metal (e.g. Pt or Ir) contaminants adhering on a semiconductor substrate or a silicon-based insulating film (e.g. silicon oxide film) formed on the semiconductor substrate and can further prevent the readhesion of the removed contaminants, and (2) a cleaning method using said cleaning solution.
The present inventors made an intensive study in order to achieve the above object. As a result, the present inventors found out that the platinum group metal contaminants present on a semiconductor substrate can be reliably removed and further the readhesion of the removed contaminants can be prevented, by using a cleaning solution which is a mixture of a metal-removing solution and a very small amount of hydrofluoric acid. The above finding has led to the completion of the present invention.
That is, the present invention is directed to a cleaning method for removal of the platinum group metal contaminants present on a semiconductor substrate, which uses a cleaning solution which is a mixture of a metal-removing solution comprising hydrochloric acid and hydrogen peroxide, and a very small amount of hydrofluoric acid, with the proviso that the concentration of hydrochloric acid in the metal-removing solution is in the range of 10 to 25 percents by weight.
The present invention is also directed to a cleaning method for removal of the platinum group metal contaminants present on a semiconductor substrate, which uses a cleaning solution which is a mixture of a metal-removing solution comprising sulfuric acid and hydrogen peroxide, and a very small amount of hydrofluoric acid.
According to the present invention, the platinum group metal contaminants present on a semiconductor substrate or the silicon-based insulating film formed on a semiconductor substrate can be removed reliably and, moreover, the readhesion of the removed contaminants can be prevented; therefore, there occur no secondary contamination in other devices.